A typical mass flow controller (MFC) is a closed-loop device that sets, measures, and controls the flow of a gas in industrial processes such as thermal and dry etching. An important part of a MFC is a sensor that measures the mass flow rate of the gas flowing through the device. The MFC compares an output signal from the sensor with a predetermined set point and adjusts a control valve to maintain the mass flow rate of the gas at the predetermined set point.
Some characteristics of a MFC tend to drift with temperature. For example, the mass flow sensor output when no gas is flowing through the device may vary with temperature, a phenomenon referred to as “zero shift.” If the temperature of the gas flowing through the MFC is known, the MFC can compensate for temperature-dependent inaccuracies such as zero shift. Thus, to make the performance of a MFC less sensitive to temperature, it is desirable for the MFC to be able to measure the gas temperature. Adding a temperature sensor to a MFC increases its complexity and cost, however.
It is thus apparent that there is a need in the art for an improved method and apparatus for measuring the temperature of a gas in a MFC.